1. Field of the Invention
The present invention relates to semiconductor light-emitting devices.
2. Background Art
The development in recent years of relatively short-wavelength semiconductor light-emitting devices, including blue light-emitting diodes (LEDs) and ultraviolet LEDs, has been flourishing. Group III-V compounds such as InGaN and AlGaN as well as Group II-VI compounds such as ZnO, materials of comparatively large band gap, are ideally suited for use in such semiconductor light-emitting devices. In one example of a semiconductor light-emitting device of this sort, an active layer containing InGaN is arranged in between an n-type cladding layer containing AlGaN, and a p-type cladding layer likewise containing AlGaN. In another example, an active layer containing ZnO is arranged in between an n-type cladding layer containing MgZnO, and a p-type cladding layer likewise containing MgZnO.
Crystals composed of a III-V compound like InGaN, or a II-VI compound like ZnO in many cases constitute hexagonal structures. In semiconductor light-emitting devices having active layers composed of such hexagonal-system crystalline compounds, the generation of spontaneous electric fields, such as piezoelectric fields, at the active layer heterointerfaces is known to occur. The larger such spontaneous electric fields are, the narrower the overlapping portions of the wave functions of the electrons and holes in the active layer become, lowering the probability that the electrons and holes will recombine. The light-emitting efficiency of such devices consequently deteriorates to the extent that spontaneous electric fields grow larger. Inasmuch as spontaneous electric fields become more extensive the thicker is an active layer, the thickness of active layers (especially well layers in quantum-well structures) is restricted for the sake of curbing degradation in light-emitting efficiency due to spontaneous electric fields. This limitation on the thickness of active layers is one factor impeding the enhancement of semiconductor light-emitting device brightness.